A population of house mice (Mus musculus) developing out of parental C x A F1 hybrids was allowed to increase to 8x optimum density in staged phases, thus permitting the survival each 200-days of sufficient new generation animals to double the size of the population at the beginning of each of seven successive 200-day periods. At about 118 days of age young mice begin to establish membership in adult social groups. An algorithm was developed from data on place of sleeping at time of capture to define episodes of range stability and instability with reference to residence within a 16-cell habitat. Each habitat cell was designed to facilitate optimal relationships among a group of 12 adults. The proportion of the total population characterized by residential stability each successive week was calculated for the socially active adults of 118-565 days of age. Residential stability dropped exponentially from 95% to 61% as crowding increased over the central 150-week history of the population, during which time the population increased to 8x optimum. However, episodes of increased residential stability were evident during each new generational cohort period as members of that cohort attained the beginning of the "prime adult" phase of behavioral development at 172 days of age. Such times of inter-period increase in residential stability will now be used to determine the major members of the group inhabiting each cell during each 200-day period. Findings on groups will then be used to analyze changes in behavior accompanying increased crowding.